This invention relates to laser systems and methods, and more particularly, to a laser rangefinder for determining the distance between a reference and a target using pulses of coherent light at varying frequencies.
Various prior art rangefinders have employed coherent light i.e. laser energy to determine target locations. These rangefinders employ a laser medium, such as neodymium YAG, to generate a laser energy pulse. A portion of the laser pulse energy is fed, through a minimum distance path, directly to the rangefinder receiver and a portion of the laser pulse energy is directed to the target to produce a reflection energy, a portion of which is also directed back to the rangefinder receiver. Fiber optic coupling is often utilized for the direct, minimum distance path transmission medium for the initial energy to the range finder receiver; however, any suitable transmission medium may be utilized.
The receiver responds to the directly fed and reflected inputs with electrical signals whose temporal characteristics represent the time required for the laser energy to travel to the target and back from the target to the rangefinder. The laser rangefinder utilizes time of arrival techniques to compute the distance to the target.
A drawback to prior art rangefinders is that the emission frequency is always narrowly centered about the primary frequency of the laser medium. This emission frequency can be detected by equipment located at the target. Consequently, an electronic countermeasure system can transmit an energy signal at the same frequency as the laser rangefinder to defeat the operation of the rangefinder.
An additional drawback, for some prior art rangefinders, exists in the manner in which the human eye reacts to energy contained within the near infrared spectrum (i.e. up to 1.4 microns). The eye does not detect, recognize, or respond to this spectrum (between 0.7 microns and 1.4 microns) of input; however, the lens of the eye will pass this spectrum with little attenuation to the retina. The lens reacts with this spectra of input, to provide considerable energy concentration to the area of the retina illuminated. In this fashion, the retina is at risk of damage.
Known atmospheric effects, where aerosols are suspended, and where temperature and humidity vary, produce varying transmission for laser energy. As these variations occur, a specific spectral frequency may experience greatly reduced transmission, while yet another specific spectral frequency may experience greatly increased transmission. Prior art laser rangefinders with fixed spectral frequency emission are unable to adjust to accommodate these atmospheric conditions.
Some prior art laser rangefinders suffer reduced performance at high pulse repetition frequencies, due in most part to thermally induced lensing in the laser medium. These rangefinders produce a temperature gradient across the laser medium due primarily to the high pump excitation requirement for the laser medium, and the inability of the coolant mechanism to extract the resultant waste heat. In view of the limited pulse repetition frequency available for these rangefinders, the available target position data rate is often inadequate to characterize the target motion.